Alteration processes of carbonaceous chondrite parent bodies

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(1) Various types of dark inclusions (DIs) in CV chondrites have been studied in detail by using scanning electron microscopy equipped with an energy dispersive X-ray spectrometer. DIs have been been thought to be primary aggregates of condensates from the solar nebula. However, from all the DIs studied our study revealed evidence that they were affected by aqueous alteration and stbsequant dehydration on the meteorite parent body. The compositional and mineralogical features of DIs indicate that they were probably derived from their host CV chondrites. These results suggest that water and heat source existed in the CV parent body and that aqueous alteration and thermal metamorphism occurred actively.(2) Ca-Al-rich inclusions (CAIs) in four Antarctic CO3 chondrites and four non-Antarctic CO3 chondrites have been studied in detail. Nepheline was found to occur in CAIs in all the CO3 chondrites. Nepheline is formed by replacing mainly melilite and anorthite, indicating it to have an alte … More ration origin. Previously the alteration was thought to have occurred by reaction with the solar nebular gas before accretion. Our study shows that the amount of nepheline increases in the same order as that of metamorphic grade of the CO chondrites. This result suggests that the alteration of CAIs is related to the metamoprhism of the host meteortes, thus it occurred on the meteorite parent body rather than in the solar nebula.(3) In order to better understand the aqueous alteration processes and conditions, we did hydrothermal alteration experiments of the Allende CV3 chondrites. Various alteration mineralogy and textures were produced that resemble those in CI and CM chondrites. Based on the results, we can understand the details of mineralogical and chemical changes that occurred during aqueous alteration of the meteorites.(4) In order to gain our knowledge about the shock histroy of carbonaceous chondrites, we performed shock recovery experiments of the Murchison CM,Yamato 791717 CO and Allende. CV chondrites and studied the recovered samples in detail by using a scanning electron microscope. The meteorite samples were shocked at 5,10,20,25,30,35, and 40 GPa. The carbonaceous chondrites of type CM and CO have not experienced shock pressures higher than 25 GPa. The experimental results indicate that chondrules were flattened at pressures higher than 10 Gpa. The degree of chondrule flattening increases in accordance with the intensity of shock pressure. The experiments also reveal mineralogical and chemical changes that occur in the pressure range between 20 and 40 GPa. Less